1. Field of the Invention
The invention relates to a rotation angle detection device that detects a rotation angle of a rotor, such as a rotor of a brushless motor.
2. Description of Related Art
In order to control a brushless motor used in an electric power steering system, or the like, currents need to be conducted through stator coils in accordance with the rotation angle of a rotor. Then, there is known a rotation angle detection device that detects the rotation angle of a rotor of a brushless motor using a detection rotor that rotates with rotation of the brushless motor. Specifically, as shown in FIG. 12, a detection rotor 101 (hereinafter, referred to as “rotor 101”) includes a cylindrical magnet 102 having a plurality of magnetic pole pairs that correspond to magnetic pole pairs of a rotor of a brushless motor. Two magnetic sensors 121 and 122 are arranged around the rotor 101 at a predetermined angular interval about the rotation axis of the rotor 101. The magnetic sensors 121 and 122 output sinusoidal signals having a predetermined phase difference. The rotation angle of the rotor 101 (the rotation angle of the rotor of the brushless motor) is detected based on these two sinusoidal signals (see Japanese Patent Application Publication No. 6-109750 (JP 6-109750 A)).
In this example, the magnet 102 has four magnetic pole pairs. That is, the magnet 102 has eight magnetic poles that are arranged at equiangular intervals. The magnetic poles are arranged at angular intervals of 45° (180° in electric angle) about the rotation axis of the rotor 101. In addition, the two magnetic sensors 121 and 122 are arranged at an angular interval of 22.5° (90° in electric angle) about the rotation axis of the rotor 101.
The direction indicated by the arrow in FIG. 12 is defined as the forward rotation direction of the detection rotor 101. Then, as the rotor 101 is rotated in the forward direction, the rotation angle of the rotor 101 increases, whereas, as the rotor 101 is rotated in the reverse direction, the rotation angle of the rotor 101 reduces. As shown in FIG. 13, the magnetic sensors 121 and 122 respectively output sinusoidal signals V1 and V2. During one period of each of the sinusoidal signals V1 and V2, the rotor 101 rotates an angle (90° (360° in electric angle)) corresponding to a single magnetic pole pair.
The angular range of one rotation of the rotor 101 is divided into four sections corresponding to the four magnetic pole pairs. The angle of the rotor 101, which is expressed on the condition that the start position of each section is 0° and the end position of each section is 360°, is termed the electric angle θe of the rotor 101.
Here, the first magnetic sensor 121 outputs the output signal V1 (=A1·sin θe) and the second magnetic sensor 122 outputs the output signal V2 (=A2·cos θe). A1 and A2 are amplitudes. If the amplitudes A1 and A2 of the respective output signals V1 and V2 are equal to each other, the electric angle θe of the rotor 101 may be obtained using both output signals V1 and V2 according to Equation 1 below.
                                                                        θ                ⁢                                                                  ⁢                e                            =                            ⁢                                                tan                                      -                    1                                                  ⁡                                  (                                      sin                    ⁢                                                                                  ⁢                    θ                    ⁢                                                                                  ⁢                                          e                      /                      cos                                        ⁢                                                                                  ⁢                    θ                    ⁢                                                                                  ⁢                    e                                    )                                                                                                        =                            ⁢                                                tan                                      -                    1                                                  ⁡                                  (                                      V                    ⁢                                                                                  ⁢                                          1                      /                      V                                        ⁢                                                                                  ⁢                    2                                    )                                                                                        Equation        ⁢                                  ⁢        1            The thus obtained electric angle θe is used to control the brushless motor.
In the above-described conventional rotation angle detection device, because the amplitudes of the output signals V1 and V2 of the respective magnetic sensors 121 and 122 fluctuate from one magnetic pole to another due to, for example, variations in magnetic force among magnetic poles, an error may occur in detecting the rotation angle of the rotor 101. Therefore, the output signals V1 and V2 of the respective magnetic sensors 121 and 122 are corrected (the amplitudes are corrected) such that the amplitudes of the output signals V1 and V2 of the respective magnetic sensors 121 and 122 are equal to each other based on the mechanical angle of the rotor 101, and then the electric angle θe of the rotor 101 is computed.
When magnetic force varies among the magnetic poles, amplitude correction values used to correct the amplitudes of the output signals V1 and V2 of the respective magnetic sensors 121 and 122 by one period or half a period in the electric angle need to be changed. Therefore, to perform such amplitude correction, it is necessary to identify the magnetic poles sensed by the magnetic sensors 121 and 122. After the rotor 101 rotates 360° in mechanical angle, it is possible to identify the magnetic poles sensed by the magnetic sensors 121 and 122 based on the differences in peak value among the magnetic poles. Therefore, it is possible to perform amplitude correction based on the magnetic poles sensed by the magnetic sensors 121 and 122. However, immediately after start-up of the brushless motor, it is not possible to identify the magnetic poles sensed by the magnetic sensors 121 and 122. Therefore, it is not possible to perform amplitude correction based on the magnetic poles sensed by the magnetic sensors 121 and 122.